‘I will never be able to ask him why’

Washington: The gunman was running and shooting. Navy Captain Christopher Mercer could hear the screaming, and the shots getting closer, outside his third-floor office. Someone had pulled the fire alarm.

He set up camp right in front of my office. He kept reloading and firing at cubicles.

Arnold was one of the Navy's pre-eminent ship designers, but he was enamoured with planes, finally living out his dream by building one with a kit in the basement of his Virginia home. An aviation-themed calendar was spread out on the desk in front of him when Alexis entered the office. Another staffer, who had been in the office a moment earlier, turned as he ran, and saw a silent Alexis pull the trigger, sending a shotgun blast into Arnold's chest.

Evacuees raise their hands as they are escorted from the scene of a shooting at the Washington Navy Yard. Photo: Reuters

That split second gave Mercer and three colleagues time to slam the door to his office and begin pulling furniture in front of it. As soon as they backed away from the door, the gunman must have turned their way. A shot ripped through the wood, Mercer recalled, splintering it at shoulder height.

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Mercer and the other three trapped employees - a young female contractor and two former Navy officers - dived under his desk.

"He set up camp right in front of my office. He kept reloading and firing at cubicles. Later, when he came back, I could see his shadow through the glass pane in my door." Mercer said in his first interview since the attack. "I've been through my share of mishaps. But this was unreal. It was just so utterly violent."

The aftermath: a family member of a Navy Yard worker who was evacuated during a shooting arrives hoping to find him at a makeshift shelter at the Nationals Park baseball stadium. Photo: Reuters

Huddled under his desk, Mercer became the closest thing that hundreds of police officers descending on the Navy Yard had to eyes and ears on Alexis as the gunman moved through the third floor of Building 197, stalking the offices of some of the Navy's most senior amphibious ship builders and buyers.

It was after 8.30 am. Mercer thought he could hear Alexis pacing through the nearly empty labyrinth of cubicles along the western side of the third floor.

Back near Arnold's office, he would later learn, the gunman stumbled upon a young woman from Mercer's staff who was crouching down between a filing cabinet and a metal support beam. Alexis pointed the shotgun at her face and pulled the trigger, she told others, but nothing happened. He ran.

Under siege: People come out from a building with their hands up after the shooting at the Washington Navy Yard. Photo: AFP

Mercer, on the floor under his desk and armed only with his BlackBerry, began firing off emails to his commanders as he heard the door to the emergency staircase behind his office wall fly open.

The gunman, it seemed, was running down the stairs, toward the alleyway adjacent to the power plant where many of Mercer's staffers had just fled. There were two shots. One of the two bullets, Mercer can only now imagine, was the shot that killed the maintenance worker standing beside Navy Commander Tim Jirus in the alley.

Minutes went by, and then several more. Mercer, who leads a group designing a new $US4 billion ($4.3 billion) aircraft carrier to ferry the joint strike fighter, felt safe enough to climb out and sit at his desk.

Died at the scene: the FBI released two photos of shooter Aaron Alexis. Photo: Supplied

He was getting bombarded with emails from staffers who had seen Arnold get shot. Mercer wanted to venture across the hallway to check on his friend. "I was tempted," Mercer said. But there was no way of knowing whether it was safe. And there was no one crying for help.

Mercer was also now in communication via email with his boss, one of the leaders of the Senior Executive Service, who was standing outside Building 197 beside a police commander relaying his account.

Stay there, they were told. Mercer nervously passed the time clicking through Microsoft Outlook, pulling up the office numbers for Arnold and others around him. He relayed the numbers to police, to tell them where the gunman had been.

Then, there was the sound of footsteps outside again. Mercer and his terrified colleagues slid back behind the desk. The could hear Alexis reloading his weapon.

"I couldn't believe he came back - to the same spot," Mercer said.

In a way, it made some sense. Most offices on the second floor of Building 197 required specialised key cards to enter. The third floor was the first place where there were nooks and crannies where someone could wander.

Halfway down the west side of the building, where the looming smokestacks of the power plant next door can be seen through the windows, the gunman began to make his own last stand, sometime around 9am.

"He was next door, through a little gypsum wall. He was moving furniture," Mercer said. Mercer knew the office number. He emailed the coordinates: 3(W)20820.

Within minutes he could hear police. And then an eruption of gunfire.

"It was a fierce, major gun battle. Bullets were flying through my office, over our heads, and kept going for minutes," Mercer said. "Then, I heard 'shooter down, shooter down'."

It was 9.25, Mercer remembered, when police pulled him and the three others from the office into the hall.

Mercer's office was a wreck: Glass from frames on the wall was shattered; the floor was littered with the confetti of bullet-shredded wallboard and paper.

Outside were two officers with flak jackets and rifles. Another officer stood guard over a discarded weapon. Shell casings covered the floor.

A few steps to the south, as they walked out of a passageway into the main north-south corridor, "we literally almost had to step over Marty," Mercer said. That was Marty Bodrog, a retired Navy officer who found a second, civilian career overseeing the design and procurement of the amphibious warships used to ferry Marines and their supplies around the world.

A day after the rampage, Mercer and the three co-workers who had huddled with him under his desk dialled into a conference call with a counsellor. They practiced breathing exercises and talked about what a normal response to such a horrific event might be like.

Later, they would start retelling their stories, the counsellor said. Much later they would try to make sense of it.